Nanostructures such as NanoWires (NW) and Carbon Nanotubes (CNT) have been identified as one of the most promising candidates to extend and even replace materials currently used in microelectronic manufacturing processes. For example, metallic CNT have been proposed as nano-electronic interconnects due to their high current carrying capacity, whereas semiconducting NanoWires (NW) have been indicated as nanoscale transistor elements due to the possibility of forming wrap-around gates and exploitation of quantum confinement effects in the narrow wires. These and similar applications cannot be fully accomplished yet since the fabrication of nanostructures still faces a variety of unsolved issues, which vary from one application to another but may, however, be similar in some aspects as discussed below.
Vapor-liquid-solid (VLS) growth is one of the most common methods used to synthesize NW connecting with other components in a semiconductor device. However, the growth temperature for NW in a chemical vapor deposition (CVD) chamber is typically around 600° C.-700° C. which is relatively high. This high temperature can readily damage the underneath device layer. Therefore, to reduce the growth temperature and at the same time to synthesize a crystalline structure is an important issue.
Liu et al. [Japanese Journal of Applied Physics Vol. 46, No. 9B, 2007, pp. 6343-6345] describes a VLS based growth technique making use of an Anodic Anodized Oxide (AAO) template and Au nanoparticles as catalyst to form silicon nanowires inside the AAO template. By using the AAO template the growth temperature could be lowered to 400° C.-550° C. because the alumina in the AAO template acts as an extra catalyst in addition to the Au catalyst particles.
Another issue is the growth of segmented NW, e.g. a NW having a first segment made of a first semiconducting material and a second segment made of a second semiconducting material. A VLS based growth technique is suitable to achieve e.g. segmented III-V nanowires, but as mentioned above this technique requires relative high growth temperatures (e.g. in the range of 600° C.-700° C.).
Still another issue is the controlled doping of the (semiconducting) NW. It is difficult to control the dopant concentration in the NW and to obtain in a NW low dopant concentrations of e.g. lower than 1017 in the nanowire or very high dopant concentrations of e.g. higher than 1018. Using the VLS technique, one can add dopant gasses to the gas mixture for growth but due to the fact that the doping concentrations in the gas mixture are very low, it is very difficult to control the dopant concentration in the gas-phase versus the incorporated dopant concentration in the nanostructure. Moreover it is hard to obtain high concentration this way.
As a conclusion there still exist a problem in the state of the art to provide an easy controllable growth method, preferably at low temperatures, to obtain mono-crystalline nanostructures which method is capable to incorporate specific concentrations of dopant elements and which method is capable of forming easily segmented nanostructures made of at least two segments made of two different semiconducting materials.